Problem: The scale on a map is 3cm : 9km. If the distance between two cities is 54km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 3cm on the map corresponds to an actual distance of 9km. An actual distance of 54km is the same as 6 $\cdot$ 9km. The distance between the two cities on the map is 6 $\cdot$ 3cm, or 18cm.